1. Field of the Invention
The present invention relates to a transfer device for an electrophotographic apparatus which prints an image based on an electrophotographic process, a transfer roller used in this transfer device, and toner used for the electrophotographic apparatus.
2. Description of the Related Art
An electrophotographic apparatus forms an electrostatic latent image on a photosensitive surface of a photosensitive drum by charging the photosensitive surface to a predetermined potential (e.g., -600 V) by a charging device and exposing the photosensitive surface by an exposure device in accordance with image data. The electrostatic latent image is developed by attaching toner on the photosensitive surface by a developing device in accordance with the formed electrostatic latent image, thereby forming a toner image corresponding to the electrostatic latent image on the surface of the photosensitive drum. This toner image is transferred to a printing sheet by a transfer device. Thus, the image is printed on the printing sheet.
Transfer devices of various types of methods are available. One of such methods is the contact transfer method. A transfer device of the contact transfer method has a transfer roller contacting a photosensitive drum. Charges having a polarity opposite to that of the charges of the toner are given to the rear surface of the printing sheet inserted between the photosensitive drum and the transfer roller. Then, the toner attaching to the photosensitive drum is transferred to the printing sheet by Coulomb force.
The electric resistance of the printing sheet largely changes in accordance with the ambient environment, especially the relative humidity. In the transfer device of the contact transfer method, when the electric resistance of a printing sheet changes, the power of Coulomb force, i.e., an amount of charges that can be given to the printing sheet also changes. Therefore, the transfer capability changes in accordance with the ambient environment. This results change in the quality of the printed image.
Generally, a transfer roller comprises a sponge layer made of a conductive urethane sponge around the outer surface of a cylindrical shaft and a resistor layer made of PVDF (polyvinylidene fluoride) around the outer surface of the sponge layer. To adhere the resistor layer on the sponge layer, a binder layer is inserted between the two layers, and the resultant structure is heated to soften the binder layer.
In the transfer roller, the resistor layer conventionally has a low adhesion strength with the sponge layer, and hence the resistor layer may undesirably peel off. When the resistor layer peels off, charges cannot be correctly given to the rear surface of the printing sheet, decreasing the transfer capability.
Further, the toner easily attaches to the surface of the transfer roller. The attaching toner causes supply of the charges to the printing sheet unstable, decreasing the transfer capability.
Furthermore, in the conventional electrophotographic apparatus, a so-called "center blank" phenomenon occurs wherein the toner at the central portions of character images and thin lines is not transferred to a printing sheet to leave a white portion on the corresponding portion of the image on the printing sheet. This is because that the distribution of the toner on the photosensitive drum is not uniform and the most toner presents at the center of the line. When the printing sheet contacts the photosensitive drum, the toner is made flat by the transfer roller. Thus, the density of the toner becomes high at the center of the line. The Coulomb force between the toners is inversely proportional to the distance therebetween. Therefore, the toners at the center of the line have strong Coulomb force which prevents the toner being transferred to the printing sheet.